1. Field of the Invention
This invention relates to a semiconductor memory device and a method of manufacturing the semiconductor memory device. More specifically, this invention relates to a nonvolatile semiconductor memory device with MOS transistors each having a floating gate and a control gate and a method of manufacturing the nonvolatile semiconductor memory device.
2. Description of the Related Art
NOR flash memories and NAND flash memories have been widely used as nonvolatile semiconductor memory devices.
In recent years, a flash memory combining the features of the NOR flash memory and the NAND flash memory has been proposed in, for example, Wei-Hua Liu, “A 2-Transistor Source-select (2TS) Flash EEPROM for 1.8-V-Only Application,” Non-Volatile Semiconductor Memory Workshop 4.1, 1997. A flash memory of this type has memory cells, each including two MOS transistors. In such a memory cell, one MOS transistor functioning as a nonvolatile memory section has a structure including a control gate and a floating gate and is connected to a bit line. The other MOS transistor, which is connected to a source line, is used to select a memory cell.
In the conventional flash memories, plural voltage value is used for operation, thus the peripheral circuit includes MOS transistors with various gate insulating film thicknesses. When an attempt is made to form the MOS transistors in the same processes, the structure of the gate electrode differs from one MOS transistor to another.